Heavy-duty vibrators are used in a number of different applications, such as vibrating rollers for compacting soil and vibrators used on concrete block making machines. The vibrators on most machines of this type consist of a shaft or shafts which have relatively large eccentric weights attached to them to create an unbalanced condition. When these shafts are rotated at relatively high speeds, the desired vibration takes place. In concrete block making machines, the mass to be vibrated is substantial and the vibrator shaft is rotated by large electric motors (of five to ten horsepower). In addition, both in soil compactors and in vibrators for use in machines such as concrete block making machines, it is necessary to start and stop the vibrators frequently during operation. To effect the starting and stopping of such vibrators, it has been the practice in the past to use mechanical friction brakes or dynamic braking by applying electric current to special windings of the motors. Because relatively high rotational speeds are used for the vibrators, the stresses created by the braking and starting up again of the vibrator shaft are significant.
In the operation of machines for making concrete blocks, the vibrators must be started and stopped at up to ten times per minute. Also, it is necessary to bring the vibrator up to its operating speed and back to rest in as short a time period as possible. When the vibrator is used in soil compacting equipment, it is necessary to stop the vibrator each time the equipment stops and changes direction, and then to restart the vibrator and bring it up to speed in a very short time period once the equipment commences moving again.
The problems which result from the on/off cycling of heavy-duty vibrators are substantial. Large amounts of power are used to start the vibrator shafts and rotate all of the masses connected therewith, such as the pulleys, belts, brake drums and the like. Then, when the vibrator shaft rotation is stopped, large amounts of energy also must be used to prevent this mass from turning. As a result, massive heat build up takes place in the electric motors due to the constant starting and stopping operation and this heat build up substantially shortens the life of such motors. If mechanical brakes are used, there is a considerable heat build up in the use of the brakes and, of course, substantial wear and tear on the brake system itself. As a consequence, the periods between brake drum replacements are relatively short and the maintenance of the system is substantial. In addition, the constant starting and stopping of the heavy load creates a significant stress on all drive belts, couplings or gears which results in a shortened life and the necessity of using heavy-duty components throughout the system. Because of the constant power surging which results when the large electric motors are constantly started and stopped in the cycle of operation, power consumption is substantial and heavy-duty power lines and the like are necessary.
In concrete block making machines, the start-up and stopping times for operating the vibrator result in nonuniform block quality. This apparently is caused by horizontal vibration components which are present during transitional operating periods.
In addition, most heavy-duty vibrator systems are capable of providing only a fixed amount or amplitude of vibration for any given rotational speed of the vibrator shaft. If a different amplitude of vibration is desired, it has been necessary in the past to stop the system and change the weights or the relative locations of the weights on the vibrator shaft. After this has been accomplished, normal operation of the vibrator then once again can be resumed. Any time subsequently that a change in the vibration amplitude is desired, the cycle of stopping the system, readjusting the weights, and starting the system up again, must be repeated.
Attempts have been made in the past to provide a hydraulically variable vibrator system which does not require the stoppage of the vibrator shaft in order to change the vibration amplitude or to eliminate vibration of the shaft entirely. The hydraulic system utilizes a sealed tank mounted on the shaft and hydraulically movable weights are located in the tank. The positions of the weights can be adjusted with respect to one another to either be placed on diametrically opposite sides of the shaft (a balanced condition) or near one another in varying degrees to impart different amounts of amplitude vibration to the shaft due to the eccentric location of the composite weights. Such a hydraulic system, however, requires complex seals and controls to effect the desired adjustments and results in a relatively expensive system.
Accordingly, it is desirable to provide a mechanical variable vibrator in which the vibration amplitude can be changed from zero (a balanced condition) to some maximum amplitude of vibration and back again in a minimum period of time while the vibrator shaft is continuously running to avoid the necessity of starting and stopping the vibrator shaft.